This invention relates to electron guns, such as used in cathode ray tubes, and particularly to an improved beam forming region for such electron guns. The invention may be incorporated into many different types of cathode ray tubes, which in turn may be incorporated into many different types of television receivers. The invention also may be incorporated into many different types of electron guns; however, in the following description, the invention is described with respect to an in-line electron gun which is used in a slit-mask line-screen cathode ray tube having a self-converging deflection yoke, which in turn is used in a television receiver.
An in-line electron gun is one designed to generate at least two, and preferably three, electron beams in a common plane and to direct the beams along convergent paths to a small area spot on the screen. A self-converging yoke is one designed with specific field nonuniformities which automatically maintain the beams converged throughout the raster scan without the need for convergence means other than the yoke itself.
The performance of an electron gun is indicated by the spot diameter of the area of a screen excited by an electron beam from the gun. It is known that such performance is degraded by spherical aberrations and space charge effects. These effects are present in various parts of an electron gun including the beam forming and beam focusing regions of the gun.
In one recently developed electron gun described in U.S. Pat. No.4,234,814, issued to Chen et al., Nov. 18, 1980, the beam forming region of an electron gun is improved by incorporation of a thick 20 mil (0.508 mm), versus 5 mil (0.127 mm), G2 screen grid electrode. Although this thick G2 electron gun produces an electron beam with a smaller spot diameter, further improvement in spot size is very desirable.